What is at 298 K for the disproportionation of ?
= ???
Half-reaction , V
0.671
–0.580
11% of national qualifiers got this right
Explanation
draftStep 1: Set up the two half-reactions with electrons balanced
,
,
We need , i.e. one PbO is oxidized to PbO2 and one PbO is reduced to Pb. Reverse half-reaction 1 (oxidation) and double half-reaction 2 (reduction) so electrons cancel (4 each):
- Oxidation: ,
- Reduction (×2): ,
Step 2: Add and simplify
Adding gives:
Cancel one Pb and note the and must combine back to to give the target reaction (). This neutralization step is not free — it has per mole of formed.
Since 4 mol of water are formed:
Step 3: Total
This matches choice (A).
Correct: properly reverses/doubles the half-reactions to balance electrons, then adds the neutralization free energy ( kJ), giving kJ.
Skips the proper half-reaction balancing entirely: treats the two given standard potentials as a simple cell ( V) and uses (from the PbO/Pb half-reaction) in , giving kJ — ignoring that the two half-reactions are written in different media (acid vs. base) and are not directly subtractable.
Correctly reverses/doubles the half-reactions and sums kJ, but forgets to scale the water-formation correction by the stoichiometric factor of 4, subtracting only one term ( kJ) instead of four, giving kJ.
Uses the same naive cell-potential subtraction as choice B ( V) but applies (from the PbO2/Pb half-reaction) instead of , giving kJ; this is simply with no correction at all for the acid/base mismatch (missing the water autoionization term).
USNCO 2024 National Part I exam, question 42. © American Chemical Society; reproduced for non-commercial practice.